Nylon yarns and products made therefrom, such as fabrics and carpeting, have long been colored by treatment with acid, cationic, or other types of dyes. Recently, yarn producers have begun incorporating colored pigments into nylon yarns to improve their resistance to degrading and fading in ultraviolet light, to provide improved resistance to chemicals and noxious fumes, and to provide permanent coloration which is not removed by washing. While some pigments can be mixed easily into the nylon without adversely affecting the filament spinning operation, most pigments--and particularly organics--cause some difficulties while being mixed into the nylon or in subsequent melt-spinning and drawing operations. In general, organic pigments may cross-link nylon, change its viscosity, increase the rate of crystallization and form spherulites resulting in increased draw tension, weakened fibers, and more filament breaks.
U.S. Pat. No. 5,108,684 ("Anton et al.") discloses processes for making stain-resistant, pigmented-colored polyamide fibers with acceptable levels of spinning performance. Those processes involve forming a random nylon copolymer made with up to 4.0 weight percent of a cationic dye additive such as 5-sulfoisophthalic acid or its salts, adding a pigment dispersed in a matrix of nylon 6 and a nylon 6/66/610 multipolymer to the random copolymer, and melt-spinning the pigment/polymer blend.
The pigment dispersions (or concentrates) used in making such fibers are typically prepared by first combining the raw pigment with the nylon multipolymer in roughly equal percentages by weight, melting and resolidifying the combination to form pigmented pellets of the multipolymer. These pellets are then remelted or "let-down" in an equal or greater amount of nylon 6, mixed thoroughly to form a uniform dispersion, resolidified, and pelletized. Polyamide fibers colored with certain pigments, however, remain very difficult to spin and draw when the pigments are dispersed in such matrices.
World Patent Publication No. 92/08829 ("Lin") discloses pigment concentrates made from a carrier polymer which is a random copolymer of hexamethylenediamine, isophthalic acid and terephthalic acid. This carrier polymer improves the spinning performance of polyamide fibers colored with many types of pigments.
Solid pigment concentrates having high melting points such as those described above are typically gravity fed to a screw melter where they are melted and mixed with fiber-forming polyamide. The resulting molten mixture is then pumped through a transfer line to a plurality of spinning positions and spun into fiber. It would be advantageous to have a pigment dispersion which is a liquid at room temperature or which melts below 100.degree. C. (i.e. does not require expensive or complex melting and pumping equipment such as a screw melter) and which therefore could easily be added to the fiber spinning process anywhere prior to the spinneret. Addition of concentrate near the spinneret minimizes waste fiber made when transitioning between colors or other additives. It also allows for multiple different fiber products to be made from a single source of fiber-forming polyamide.